The peripheral blood lymphocytes (PBL) from aging, normal individuals and patients with rheumatoid arthritis (RA) were evaluated for reactivity in a mixed lymphocyte response (MLR) and a Con A induced suppressor cell assay. The effect of incubation of cells with thymosin Fr. 5 was also investigated. No differences were noted in the mixed lymphocyte response as a function of age (range equals 20-70 years of age) or autoimmune disease. However, when the PBL's from these two groups were studied for the induction of suppressor cells using the Con A assay, significant differences were noted; 1) Cells from RA patients when preincubated wtth Con A induced substantially more suppression of an MLR response than did normal PBL. Furthermore, when these cells were preincubated with thymosin Fr. 5, in the absence of Con A, similar suppression was observed with the patients' cells. These results suggest that PBL of autoimmune RA patients have more suppressor cell activity, as well as thymosin-inducible precursor suppressor cells.